On 28 Jun 2013 at 13:48, Matt Rhys-Roberts wrote:

> In the world of 4-20mA instrumentation, is there any such thing as a=20
> typical or ideal input resistor at the receiver end?
>=20
> I notice a few diagrams using a 250R res across the Rx input terminals,=20
> which seems a default value. To some extent I guess it can be altered to=
=20
> provide the required voltage from the current range.
>=20
> It seems that I need to specify enough voltage at the Tx end, to=20
> overcome both the cable's length and the Rx's Zin. Both these=20
> resistances are a little undefined to me at this point.
>=20
> Any personal experiences about this please?

Yes, apologies in advance as I may sound like a teacher, but...

The ideal (impossible) current loop receiver would have zero resistance, cr=
eating no=20
voltage drop in the current loop.

In reality common practice is to insert a resistor in series with the curre=
nt loop and=20
measure the voltage across it, as it is proportional to current (Ohms law).=
 The=20
resistor value should be kept as low as possible to allowing multiple recei=
vers to be=20
inserted into the current loop before the voltage drop in the loop approach=
es the=20
maximum voltage the current loop transmitter can supply.

You can see that 250 Ohms gives precisely 5V at 20mA (can be fed straight i=
n to a=20
5V ADC input with no scaling) which is why it is so common. In practice a 5=
V drop=20
(or 2 or 3 such drops) in a 4-20mA current loop can be perfectly acceptable=
, as a=20
typcial 4-20mA transmitter may operate from a 24V supply.

In the receiver it is also not uncommon to add signal conditioning circuitr=
y to remove=20
the zero offset, such that instead of 4-20mA converting to 1-5V you get 4-2=
0mA=20
converting to 0-5V and so maximising the resolution of the ADC.

These days a 5V input ADC is not as common as it used to be, what with 3.3V=
=20
micros etc. In the past I've used something like a precision 50 Ohm resisto=
r and=20
amplified the voltage to suit the ADC. In other words, yes, you can use wha=
tever=20
value resistor you like.

Note that if/when multiple receivers are used they must have floating power=
=20
supplies or isolated inputs. Having said that, many typical applications ha=
ve only=20
one transmitter and one receiver, in which case they can share a common gro=
und.

Proabably you knew all that, but hope it helps others out there~!
--=20
Brent Brown, Electronic Design Solutions
9 Titoki Place, Pukete,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brown@clear.net.nz



--=20
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